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October 23, 2007

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An Update Regarding the HVTN 502 and HVTN 503
HIV Vaccine Trials

On September 21, 2007, the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH), the pharmaceutical company Merck & Co. Inc., and the NIAID-funded HIV Vaccine Trials Network (HVTN) announced that immunizations in the HIV vaccine clinical trial known as the STEP study—also referred to as the HVTN 502 or the Merck V520-023 study—would be discontinued (see Immunizations Are Discontinued in Two HIV Vaccine Trials). The decision was based on recommendations made by an independent Data and Safety Monitoring Board (DSMB), which concluded that the vaccine could neither prevent HIV infection nor reduce the amount of virus in those who became infected with HIV.

As a result of those findings, at that time immunizations and enrollment in a separate clinical trial in South Africa (the “Phambili” or HVTN 503 study) involving the same Merck candidate HIV vaccine were paused to allow for further analysis of the STEP findings. Based on that additional review of the STEP data, a separate, independent DSMB for the Phambili trial has now concluded that there is no basis for anticipating more favorable results in the South African clinical trial. Therefore, the HVTN 503 oversight committee has permanently suspended immunizations and enrollment in the study. The Phambili DSMB also recommended that HVTN 503 volunteers be told whether they received the vaccine or placebo, be strongly encouraged to return to their study sites for protocol-related tests, and be counseled about the possibility that those who received the vaccine may have an increased susceptibility to acquiring HIV infections. Volunteers in the STEP study also will be counseled about this possibility, and discussions are under way to define the details of continued follow-up for the STEP volunteers, including when those participants will be told whether they received the vaccine or placebo.

Detailed analyses of the available data are being conducted, including analyses to better understand if there may be an increased susceptibility to acquiring HIV infection among those volunteers who received the vaccine. The vaccine itself cannot cause HIV infection. Volunteers who became infected with HIV during the trials will receive access to appropriate medical treatment and care.

NIAID, Merck and the HVTN are moving quickly to broadly disseminate the available information from the STEP study. The first detailed review of the STEP data will occur on November 7, 2007, at a special open scientific session of the HVTN in Seattle. As space is limited, registration is required at

For additional information about the HVTN 502 and HVTN 503 studies, see the related Questions and Answers.

Media inquiries can be directed to the NIAID News and Public Information Branch at 301-402-1663,

NIAID conducts and supports research—at NIH, throughout the United States, and worldwide—to study the causes of infectious and immune-mediated diseases, and to develop better means of preventing, diagnosing and treating these illnesses. News releases, fact sheets and other NIAID-related materials are available on the NIAID Web site at

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit

NIH...Turning Discovery Into Health ®

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Last Updated October 22, 2007